I just use hand signals. Always works. Except when I don't use them, which means I am going straight. That seems to confuse the cagers. I think there thought processes goes like this:

"Damn, there is a cyclist" (if they notice me at all)

"Both of his hands are on the bar, I wonder what he is going to do" (if he is going slow enough to even know where my hands are)

"I'll just assume he is going to turn" at which point their assumption causes them to cut me off.

You know, I think I am giving them far too much credit. I think its more like this.

"Uh...wtf is that? 5 points for running it over"